Schottky diode is an important power device and used extensively as output rectifiers in switching-mode power supplies and in other high-speed power switching applications, such as motor drives, switching of communication device, industry automation and electronic automation and so on. The power devices are usually required characteristics of carrying large forward current, sustaining high reverse bias, such as 100 volt or above, and minimizing the reverse leakage current.
A number of power rectifiers have been used to provide high current and reverse blocking characteristics. An exemplary method to form a Schottky barrier diode is disclosed by Kanemaru et al in U.S. Pat. No. 6,483,164. the processes are shown in FIG. 1A to FIG. 1C. Referring to FIG. 1A, a semiconductor substrate having an n+ doped layer 10 and an n drift layer 20 extended to a first surface 20A is prepared. An oxide layer 25 is then formed on the first surface 20A. Afterward, referring to FIG. 1B, the oxide layer 25 is patterned to define positions of guard ring 35 at the termination region. Guard ring regions 35 doped with p-type impurities are then buried into n drift layer 20 by boron ion implantation or diffusion using boron nitride film as a source. Thereafter, a thermal oxidation is then performed to drive in the boron impurities into silicon substrate. At the same time, an oxide layer 30 is grown on the first surface and thickened the oxide layer 25. Thereafter, a second photoresist pattern 40 using to define an anode contact region is then coated on the resultant surface.
Referring to FIG. 1C, a wet etch is then performed to remove those exposed oxide layers 30, 25 by using the photoresist pattern 40 as a mask. Portions of p+ guard ring regions 35 and the n drift layer 20 in between are thus exposed. After removing photoresist pattern 40, a Schottky barrier metal layer 50 is then formed on the resultant surface. Thereafter, a third photoresist (not shown) and an etch steps are then performed to defied the Schottky barrier metal layer 50. After the layers formed on the backside surface during forgoing step are removed, a metal layer 60 is then formed, which is served as a cathode.
The conventional Schottky rectifier with guard rings 35 may require at least three photo masks. Thus, an object of the present method is to improve the breakdown voltage and simplify the processes. According to the present invention, only two photo masks are used.